When a shaft is to be coupled to a member having a circular shape or another shape so that these members are not relatively rotated, for example, a structure may be employed in which a key is fitted into key ways respectively formed in the shaft and a boss disposed on the member. This structure may be employed in a case such as that where the member is a pulley of a rotation transmission mechanism, e.g., a pulley used in an automobile or that used in usual industrial equipment, and the shaft serving as a rotating shaft is coupled to the pulley.
When the member is to be made of metal, the boss is usually formed by casting the member.
The fitted key should not be disengaged from the key ways during a period when the pulley and the shaft are rotated. Consequently, the key way formed in the boss and the key which is to be fitted into the key way are required to have very high processing and dimensional accuracies. When a coupling structure in which a key is fitted into key ways respectively formed in a shaft and a boss is employed, therefore, there arises a problem in that the burden in cost is large.